1. Field of the Invention
The instant invention relates to discharge nozzles adapted for use in injection molding machines for plastic materials. It relates, more particularly, to such nozzles equipped with integral valves for the release of the plastic material from the pressure chamber of the molding machine into the mold sprue, through the nozzle.
2. Description of the Prior Art
In the prior art it has been known to provide for injection molding of plastic parts by means of a machine in which an injection nozzle is aligned with the orifice of the mold sprue bushing and in which the injection cycle is initiated and terminated by relative, reciprocal motion between the mold and the nozzle. In such machines the mold acts as a cam entraining the nozzle, acting as a tappet, into operating a valve which is interposed between the chamber containing the molten plastic composition and the discharge orifice of the nozzle.
Such devices have a number of drawbacks such as, for example, relative motion requires guides and actuators with inherent inaccuracies, and the drive mechanisms and lock-up mechanisms which ensure sealing between the nozzle and sprue often induce vibrations. The seal between the nozzle and the sprue may be adversely affected by such vibrations and mechanical imperfections. Importantly, once injection has ceased, the retraction of the nozzle permits backflow of plastic through the sprue so that plastic material may be lost from the nozzle and represent a loss, a potential hazard to workmen and downtime of the equipment during cleanup.
It is, therefore, a primary object of the invention to overcome the aforementioned disadvantages associated with the injection nozzles of the prior art and to teach the construction of such devices for preventing drip of plastic due to inadequate seals.
It is a further object of the invention to provide such improved injection nozzles at a reasonable cost and in a form convenient in manufacture and use.